Stolen iPhone and the changing Apple ID password.
My son had his iPhone stolen. Unfortunately, he did not have Find My iPhone enabled, so we followed the relevant steps on the apple support page "If your iPhone, iPad, or iPod touch is lost or stolen" https://support.apple.com/en-au/HT201472.
The first thing we did was to change his Apple ID password via his mac. As soon as we did that we tried to sign back in to iCloud on his mac with the new password, but couldn't because the two-factor authentication code was being sent to his iPhone ... which was stolen!
The only way for him to regain access to icloud is to recover his Apple ID via iforgot.apple.com, which we have just been notified will take 2 weeks!
This is incredibly frustrating, especially since we followed the instructions on the Apple Support page to the letter 😟
So ... if anyone is in a similar boat, make sure you change the phone number that is to be used for the authentication BEFORE you change your password.